1. Field of the Invention
The present invention relates to a quantum communication device, more particularly, to a free-space quantum communication device with atomic filters.
2. Description of the Related Art
It is known that cryptographic communication is directed to an information transmission mode in which information can be exchanged between two communication parties under a top-secret way. And many countries are paying more attention to the study of the cryptographic communications. Nowadays, cryptography has become an important means for safeguarding information exchange and is widely used by governments, banks, enterprises and individuals. With the rapid development and spread of the computer network and the electronic commerce, people have attached more and more importance on the security of the cryptographic communication system. Due to dramatically rapid development of the computer technology, it is more and more easier for a cryptography to be decoded by an intruder. Accordingly, the classical cryptography has been proven to have its limitation in use. Recently, quantum cryptography has been proposed as a new technology of cryptography in which the security of the information transmission is based on the Heisenberg uncertainty principle in the filed of quantum mechanics. There is no doubt that the quantum communication has become a novel technology in the field of information exchange. In view of the rapid development and wide application of it, people have paid more and more attention on the quantum communication.
Quantum communication uses single photon as carrier of transmission information. Under the condition of free-space communications, birefringence phenomenon for optical spectrum does not occur in the atmosphere. Consequently, photons in polarization states can be transmitted through the atmosphere so that fidelity of transmission of photon states can be achieved.
However, there are two problems that has not been solved until now. One is the signal turbulence of single photon in the transmission medium. The other one relates to detection of single photon under strong background light. During experimental demonstrations of free-space quantum communication based on photon polarization coding, the background light noise is normally filtered by narrow line-width interference filter or by use of an optical-fiber space filtering method. In this way, the above problems are partially solved, thus the above quantum communication can be performed well either at night or daytime.
For example, quantum keys were successfully distributed across the bordering area between Germany and Austria in October of 2002. The transmission rate of the raw key reaches 1.5-2 kHz while error rate is 5%. Further, the quantum keys were distributed across a distance of 23.4 kilometers which is the farthest distance in experimental demonstrations of free-space quantum key distribution. It is proven that there is great possibility for quantum communication to be put into practice.
Free-space quantum communication gains broad perspective on distribution of security keys between earth and satellite or among satellites by using near-earth orbiting satellites and on establishing a global quantum key distribution network. It has achieved a successful distribution with a transmission loss of 27 db. If detection efficiency is further improved and the background light noise is reduced so that transmission loss of 33 db can be endured by the signal channels, it is possible to carry out quantum key distributions across 500-1000 kilometers between the near-earth orbiting satellites.
In usual quantum communication device, the background light noise in free space is filtered generally by an interference filter before faint laser pulse sequence of random polarization coding is incident onto a beam splitter. Because the interference filter usually has a relatively wide pass-band width, noise rejection ratio of the background light is low. Therefore, error rate generated by background light noise in the free-space quantum communication is relatively high.
The atomic filter has the characteristics of high transmission rate, ultra-narrow pass-band width, large acceptance angle and quick response time. Further, the operational frequency is able to be tuned in a predetermined range. As such, the method of atomic filtering is widely used in laser communications, lidar and high-speed modulation of light. Atmosphere laser communication experiment is carried out under the condition that a relatively strong incident laser light with light power of magnitude order of mW/cm2 or μW/cm2 is used. In free-space quantum communication, however, the signals are transmitted in a manner of fidelity transmission by means of faint laser pulse on the single photon level.